Diagnostic tests, which are performed outside a normal laboratory environment and often without special instruments, have over the past few years become extremely important. Many of these tests, for example, detecting the presence of choriongonadotropin in urine in order to detect pregnancy, are in the nature of rapid immunological tests.
In recent years, the immunochromatographic test in particular has become a means of rapid immunological diagnostics. This method is characterized by the use of a porous material, such as nitrocellulose attached to a plastic film, as a medium, to which a narrow detection zone of biochemically active binder reagents are attached by absorption or in some other way. As the test proceeds, the binder reagent binds moving analytes driven by the capillary forces in the medium.
Known immunoassay methods can be roughly divided into two main types. In the so-called sandwich type, the analyte to be measured is sandwiched as a detection zone between two specific binder molecules, one of which is attached to a solid material and the other is marked with a radioactive, fluorescent, coloured, or other marker. There can be one or several detection zones. Both of the binder reagents are usually biochemically antibodies.
The analyte contained in the sample is detected by bringing the sample into contact with the detection zone together with a marked binder reagent that is also analysis specific, in which case the analyte forms a layer on the detection zone between the binder reagent attached to the detection zone and the marked binder reagent (the ‘sandwich principle’). The marked binder reagent may be coloured, which can be achieved by marking the binder reagent with microparticles, or with a colloidal metallic or non-metallic salt.
In the so-called competitive type of assay, the marked analyte competes with an analyte contained in the sample, for binding with a binder reagent. In this system, the amount of the measured marker bound in the binder reagent is inversely proportional to the amount of the analyte contained in the sample.
A third main type, so-called homogeneous assay, is represented by homogeneous enzyme-immunoassay, which is disclosed in the patent publication U.S. Pat. No. 3,817,834. Another technique, exploiting a steric hindrance, is Steric Hindrance Enzyme Immunoassay (SHEIA). In this method too, the analyte weakens, instead of amplifying, the signal being measured. As such, the use of a porous matrix as a migration base and the attachment of a specific binding reagent, such as an antibody, to it, was known already in the 1980s.
The interpretation of the results of such tests is generally based on the visual examination of the migration base, for example, in the pregnancy test, the observation, against the migration base, of the line that forms. In some tests, the visual effect that arises is so weak that photometric devices must be used to examine it. It is difficult to convert these assay results into an electronic form for further analysis.
Antibodies marked with magnetic particles are widely used in immunographic assay. The known apparatuses for detecting them are bridge-type solutions, in which a coil is used to examine the magnetic force emitted by the analyte on the migration base. U.S. Pat. No. 6,110,660 discloses an apparatus of this type, which, however, has a complicated circuit.
In the patent literature, reference to the use of magnetic particles in immunoassay is also made in patent publications such as U.S. Pat. Nos. 4,628,037, 5,252,493, 5,238,811, 5,993,740, 6,046,585, 6,150,181, and EP-386690A1. In the method of publication U.S. Pat. No. 6,046,585, a sample of the analyte forms a magnetic dipole, in which a varying magnetic field is induced by means of an excitation signal. The ends of the dipole form their own electrical fields, which are detected by means of detector coils and electronics connected to them.
To date, immunochromatography tests have been mainly applied to the detection of protein analytes. These are relatively large molecules, for the detection of which a test operating on the sandwich principle described above is suitable. However, the molecular size of many interesting analytes, such as steroids, pharmaceuticals, pesticides, etc., is so small that the sandwich principle cannot generally be applied to their detection. The competitive principle has been used to assay such analytes.